JavaScript is sometimes called the lingua franca of the Internet, because it is cross-platform and supported by many devices. It is also a loosely-structured language, which makes it possible for users to craft solutions for many types of applications.
Sometimes an entire application is built in JavaScript. Frequently, JavaScript provides a client-side frontend for applications built with Symfony, content management systems, such as Drupal, and other backend frameworks.
Node.js is server-side JavaScript, and provides a web server as an alternative to Apache, IIS, Nginx, and other traditional web servers.